The use of radiation therapy to treat cancer is well known. Radiation therapy (radiotherapy) involves directing a beam of high energy particles such as electrons, protons, or heavy ions into a target volume (e.g., a tumor or lesion) in a patient.
Before the patient is treated with radiation, a treatment plan specific to that patient is developed. The plan defines various aspects of the radiotherapy using simulations and optimizations based on past experiences. For example, for intensity modulated particle therapy (IMPT), the plan can specify the appropriate beam type and the appropriate beam energy. Other parts of the plan can specify, for example, the angle of the beam relative to the patient/target volume, the beam shape, and the like. In general, the purpose of the treatment plan is to deliver sufficient radiation to the target volume while minimizing the exposure of surrounding healthy tissue to radiation.
Existing IMPT dose delivery techniques utilize raster scanning that takes advantage of the well-known Bragg peak characteristic of a mono-energetic particle (e.g., proton) beam. By scanning the beam in the X and Y directions, a “layer” of dose can be “painted” within the target volume. Subsequent layers are painted in overlapping raster scan patterns using particles with a different energy that would thus stop at a different range (distance). Such scan patterns usually start at the most distal edge of the planning target volume and each subsequent layer is delivered, after a pause to change the beam energy, to a lesser range thus creating a Spread Out Bragg Peak (SOBP), until the final layer is delivered to the proximal edge of the planning target volume.
A fundamental concern during radiation therapy is that the target volume might move during dose delivery (e.g., due to the patient moving, breathing, etc.). Movement during dose delivery can inadvertently place healthy tissue in the path of the radiation intended for the target volume. Although it is theoretically possible for the raster scan pattern to track in-plane motion of the target volume, by superimposing the raster scan pattern with the instantaneous two-dimensional (X-Y) vector corresponding to that motion, any out-of-plane motions (particularly those of normal healthy structures proximal to the target) can introduce motion-related uncertainties that in turn can create dose overlaps (“hot spots”) or, even worse, gaps (“cold spots”) within the target volume.
A recent radiobiology study has demonstrated an advantageous effectiveness in sparing normal, healthy tissue from damage by delivering an entire, relatively high therapeutic radiation dose within a single short period of time (e.g., less than one second). However, in conventional raster-scanned IMPT, because dose delivery along each ray passing through the patient occurs successively at different points in time in the scan pattern and is thus spread out over time, the unavoidable dose that is delivered to the normal healthy structures is also spread out over time. Therefore, the radiobiological tissue-sparing effects reported in the aforementioned study are not realized using existing IMPT techniques.
Furthermore, contemporary radiation therapy delivery systems include dipole electromagnets and scanning magnets. The dipole magnets (often referred to as “bending magnets”) direct (e.g., bend) the particle beam in a direction toward a nozzle, and the scanning magnets steer (deflect or scan) the beam in the X and Y directions. The dipole magnets typically utilize massive ferromagnetic return paths and therefore have a much slower magnetic hysteresis relative to the scanning magnets. That is, it takes much longer to change (increase or decrease) the level of magnetism in the dipole bending magnets than it does to steer the beam using the scanning magnets during IMPT delivery. Also, the relative slowness of varying the magnetic fields of the dipole bending magnets is the primary reason that existing IMPT systems utilize a method of scanning dose one layer at a time. The time spent changing the magnetic strength of the dipole magnets in order to change the incident beam energy constitutes a significant portion of the time required to deliver an IMPT therapy dose. Considering the comfort of the patient, for example, shorter radiotherapy sessions are highly preferred. Thus, the reliance on magnets, particularly the use of the dipole bending magnets, for adjusting particle beams is an obstacle to realizing the benefits of using relatively high therapeutic radiation doses within a very short period of time for dose delivery in radiotherapy.